Piezoelectric transducers



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, zr f) m A rrae/vzrs United States Patent 2,786,899 PIEZOELECTRIC TRANSDUCERS Richard W. Carlisle, Elmsford, N. Y}, assignor-to ono: tone Corporation, Elmsford, N. Y.-, a corporation of New York Application August 2, 1951, Serial No. 239,905

8 Claims. (Cl. 179-110) This invention relates-to piezoelectric transducers and more particularly, to electroraooustic devices such as microphones or the like operating with a piezoelectric transducer element.

The present invention Was conceived as part of an effort to devise a subminiature hearing aid microphone of a sensitivity comparable to the sensitivity of conventional hearing aid microphones but having only about the same small dimensions asthe spring clip of the type used for attaching the encased miniature hearing aid amplifier microphone unit to-a garment of the user.

Among the objects of the invention is a novel piezoelectric bilamirrate transducer unit which though confined to only a fraction of the volume of asmall hearing aid microphone unit of the foregoing type will operate with about the same sensitivity as conventional hearing aid microphones heretofore used in hearing aid amplifier microphone units.

Another object of the invention is amicrophone of the foregoing type which is highly effective in suppressing cloth-rubbing noise.

A further object of the invention is a novel subminiature hearing aid microphone of the foregoing type operating with a moisture. susceptible piezoelectric unitland which is driven by a vibratory diaphragm while housed in a vapor-proof sealed enclosure having a limp barrier wall across which the vibratory diaphragm transmits the vibratory forces. to the enclosed piezoelectric unit.

The foregoing andother objects of the invention will be best understood from the following description of exemplifications thereof, reference being had to the accompanying drawings wherein;

Fig. 1 is an elevational view, with the front cover broken away along lines 11 of Fig. 3, one form of the microphone of the invention, with the parts of the interior exposed;

Fig. 2 is a cross sectional view of the microphone along lines 2-2 of Fig. 1;

Figs. 3 and '4 are cross-sectional views of the microphone along lines 3-3, and 4-4,,respeotively, of Fig. 1;

Fig. 5 is an elevational view :of apiezoelectric bilaminate unit of the type shown in Figs. 1 and 2, and exemplifying one form of the invention;

Fig. 6 is a sideview of the piezoelectric unit of :Fig. 5 with some of the dimensionsex-aggerated for the sake-of clarity;

Fig. 6-A is agreatly enlarged;cro ss ,-sectional view similar to Fig. 5 of a portion of the middle part of the piezoelectric unit as seen at 5-1 in Fig. 5;

Fig. 7 is a view similar to.-Fig. 5 ofanouher form of a piezoelectric bilaminate unit of the invention.

Most hard-of-hearingor deafened persons are very sensitive and seek to conceal their impairment. Accordingly, a practical hearing aid-must be light, small and compact enough so that it may be worn comfortably andi'nconspicuously hidden behind a garment portion of the body of the user. In addition, it must be. simple, and fool-proof in operation and should require lit tle attention so .as to free the user from mental and physical strain and annoyance. It is a general practice to makethe hearing aid microphone an essential part of the amplifier unit, and house it directly behind a sound pervious wall portion of the front wall of the flat amplifier casing, such amplifier unit being, as a rule, worn behind a garment portionof the user. in addition, microphones which are used as a part-of a high gain amplifier hearing aid, have to operateundef conditions utterly different from those encountered in other microphone applications. This is due to the fact that the microphone has to be worn on or more orless hidden under the clothing of the user where itis exposed to humidity or moisture which greatly affects its pefform= ance, particularly, if the microphone transducer element is formed of piezoelectric Rochelle salt or similarcrystal materials which are highly susceptible to moisture and the performance of which is greatly impaired thereby.

Furthermore, it is essential to keep the space occupied by the microphone to a minimum, while at the same time providing for performance with a desired high sensitivity over the principal part of the audio-frequency rangeessential for intelligible reproduction of speech.

These requirements impose very strict limitations on the construction of such hearing aid microphones since in order to obtain high sensitivity, its vibro-electric transducer element must be driven by a relatively large vibratory diaphragm exposed to sound propagated in thesurrounding atmosphere. I

The microphone of the PIfiSfCIltll'lVgClltlOIl combines a vibratory diaphragm with a novel type of piezoelectric bilaminate in such manner as to make it PQSSlblGIO limit its overall volume to the space occupied by the springclip by means of which a conventional hearing aid microphone combination unit of the prior art was ordinarily attached to the garment of the user.

Although a microphone of the invention may be housed in the interior of the casing enclosing the other elements of the hearing aid amplifier and associated parts, with or without the earphone receiver, there will be here described one form of such microphone which constitutes part ofja small spring clip overlying the front wall of a tiny compact hearing aid amplifier casing by means of which such jarnplifier unit is ordinarily attached to the garmentrof the user.

Referring to Figs. 1 to 3, there isshown in agreatly enlarged side view, a part of a relatively thin flat casing 8 of a conventional hearing aid amplifier unit which encloses substantially all elements of the hearing aid, but with the hearing aid microphone 30 disposed in front of the flat front wall 9 of theamplifier casing 8.

The amplifier casing '8 with the associated amplifier and the other cooperating elements of the hearing aid enclosed therein may have the form of any of the known prior art miniature hearing aids and may be of the type described, for instance, in the co-pending application Ser. No. 129,374 filed November 25, 1949, now Patent No. 2,699,471, granted January 11, 1955, by R. W. Carlisle et a1.

The microphone 30 is housed within a microphone-casing 10 formed by the flexible spring clip 11, shown in the form of a strip extending adjacent the upper portion of the amplifier casing wall9 anda cooperating backing strip ls joined thereto to form the front and rear walls, respectively, of the microphone casing 10.

The clip forming frontwall 11 of the micropl' one casing 10 is made frorn flexible spring sheet metal,. such as a strip of stainless steel, and itsupper end ispaifixed to or extends from the upper part of :the amplifiercasing 8. In the form shown, the upper region of ,the clip strip 11 is provided with two side wings 13 abutting against the front wall 9 of theamplifier casing 8 and {an upper strip extension 14 secured at 15 in overlapping relation to a recessed wall portion of the upper border wall 6 f the amplifier casing 8. The front wall 9 of the amplifier casing 8 with its border Wall 6 may be likewise formed of sheet metal. The clip wings 13 are shown provided with hook extensions 17 shaped to enter longitudinal slits formed in the casing front wall 9 so as to be hooked in and retained behind the edge of the easin'g wall bordering the respective slits 5 of the casing wall. With this arrangement the clip 11 may be readily affixed to the metallic front wall 9 of the casing, by inserting its two side-wing hooks 17 into the two slits 5 of the amplifier-casing front wall 9 and thereafter securing the rear portion of the upper clip extension 14 of the clip 11 to the underlying recessed border wall portion 6 of the amplifier casing by soldering or welding or by a screw 15.

The lower end of the resilient and flexible clip arm 11 has an inwardly bent nose portion 18 which is biased by the restoring forces of the elastically deformed major length of the clip arm 11 to press an underlying garment portion 181 of the user against the front wall 19 ofthe amplifier casing 8 and thereby holding it attach'ed, in the desired position, to the garment.

The rear wall 15 of the microphone housing is shown likewise formed of sheet metal and has two forwardly extending wings 21 terminating slightly behind the facing springy clip wall 11 to provide therebetween air gaps 22 through which sound is propagated form the surrounding space into the interior of the microphone housing 10. A lower tail extension 24 of the microphone-casing rear wall and forward nose portions 23 of its two side wings 22 are arranged to be seated and retained between the inwardly bent upper edge of the nose portion 18 of the spring clip and the facing portion of the overlying clip arm 11, thus providing an articulated junction between the lower ends of the clip arm 11 and its associated'rear wall 15.

The upper end of the rear wall 15 is likewise joined by an articulated junction to an adjacent upper region f 1'2of the clip arm 11 so that the two walls 11 and 15 of the microphone housing shall be retained in their proper spacedpositio'n while permitting the resilient clip strip forming front wall 11 of the microphone housing to flex away from the amplifier casing wall 9 and permit insertion 'or'removal of a' garment portion 13-1 from "between the clip nose 18 and the facing front wall 9 of the amplifier casing 8.

j-Inthe form shown, the upper junction connection between the clip wall 11 and its rear wall 15 is provided by pivot tongue portions 26 struck or'cut out of the upper side wings-13 of the spring clip 11 and bent inwardly to enterunderlying pivot junction holes or openings 27 formed in the side wings 21 of the rear wall 15 of'the'microphone' housing. With this arrangement, the

rear wall 15 maybe readily assembled in its properly spaced position against the back of the resilient flexible clip arm wall 11, by first inserting the lower junction portion 24 of the rear wall 15 into the space behind the clip nose 18, and thereafter springing the junction tongue 26 of the upper side wings 13 of the clip arm 11 into the junction holes 27 of the side wings 21 of the rear 'wall' 15'0f the microphone housing. The nose portion- 18 ofthe resilient flexible clip wall 11 is shown stiffened by providing the bottom portion of the spring clip strip 11' with two wing elements 19 forming side walls enclosing of the inwardly bent nose portion 18 of the clip strip 11.

Within the elongated interior space of the microphone housing, forming part of the flexible spring clip arm 11, is held suspended the microphone 30. The microphone 'unit 30 shown comprises a generally elongated shallow outer cavity casing 31 having a narrow border wall 32 provided with a peripheral flange 33. To the peripheral flange 33 of the microphone cavity casing31 is secured the periphery 34 of a vibratory diaphragm 35 which is arranged to impart its vibratory driving motion to a piezoelectric transducer unit 50 held in its operative position adjacent the cavity casing wall 31.

The piezoelectric transducer unit 50 shown in Figs. 1, 2, 4, 5, 6, 6-A, represents one form of the invention. It comprises a piezoelectric bilaminate formed of two layers 51, 52 of piezoelectric material (Figs. 5, 6 and 6-A) joined to operate as a piezoelectric bender transducer which when bent relatively to its major longitudinal axis, indicated by dash-dot line 60, will generate across electrodes secured to the outer surfaces of the bilaminate 50 a corresponding electric output voltage. The piezoelectric layers 51, 52 have a generally diamondlike quadrangular shape and they constitute a vibratory transducer structure which transduces mechanical signals into electric signals and vice versa.

The piezoelectric material of the bilaminate layers 51, 52 may be formed of polycrystalline titanate material which is rendered piezoelectric in accordance with the disclosure in the co-pending application of J. S. Crownover et al., Serial No. 772,934, filed September 9, 1947, as a continuation-in-part of application Serial No. 694,- 386, filed August 31, 1946, and now abandoned, and application Serial No. 727,152, filed February 7, 1947. Alternatively, the piezoelectric material of the bilaminate layers 51, 52 may be formed of conventional piezoelectric crystal material, such as Rochelle salt or ammonium dihydrogen phosphate crystal material.

The specific microphone unit 30 shown was designed for operation with a bilaminate 50 of Rochelle salt crystal material which gives a higher voltage output at a lower impedance than obtainable with other commercially produced piezoelectric materials.

In general, the piezoelectric material of portions of a piezoelectric transducer which are subject to the greatest stress generate a greater output voltage. For utmost effectiveness it is very desirable that substantially all the piezoelectric material of such transducer should be subjected to a substantially equal stress when performing transducing operations. This requirement is of particular importance in applications, such as a hearing aid microphone, in which the dimensions of the piezoelectric transducer unit must be kept down to a very small volume.

In the specific form shown, the piezoelectric bilaminate 50 forms a generally diamondrshaped fiat transducer unit having its major longitudinal axis 60 extending diagonally between two relatively far corner portions 53 and a minor transverse axis 60-1 extending diagonally between its two much closer corner portions 54 (Figs. 5, 6). i

The piezoelectric bilaminate 50 shown is designed according to the invention so that when its intermediate portion 55 extending between its two diagonally close corners is bent relatively to its far apart corners 53, the stresses throughout substantially the entire piezoelectric material of the two laminate layers 51,52 will be equalized thus assuring optimum piezoelectric transducer action and optimum output characteristics as a mechanoelectric transducer. p

In the particular type of piezoelectric transducer 50 shown (Figs. 5' and 6), the intermediate'portion 55 of "leading to their far 'ends'53 ot' the bilaminate and also by additionally stiffening its intermediate region 55 which is driven with maximumdisplacement.

The greater transverse stilfness of the intermediate 're- 'gion is secured by giving the layers 51, 52 a generally rectangular diamond shape, the greater width across the transverse axis 60 providing for gradual decrease of the stilfness toward the opposite far ends 53 along the major axis 60 along which the bilaminate is subjected to a vibratory bending motion.

For additional control of the stilfness, one of the opposite surfaces of each of the two piezoelectric layers 51, 52 is provided with metallic surface electrodes 56, 57 only along the wide intermediate region 55 which is driven with a maximum displacement, thereby increasing the stiffness of the intermediate region of each of the piezoelectric layers 51, 52 at the portions driven at maximum displacement. It should be noted that when applying surface electrodes to suitably cut piezoelectric layers of Rochelle salt crystal material, it is conventional practice to first apply a minute surface stratum of graphite to the exposed surfaces of the Rochelle salt crystal layer. Thereupon a metallic surface layer is applied to the so-graphitecoated surface of the crystal layer, for instance, by coating it with a thin layer of cement which is saturated and loaded with a fine metallic silver powder such as silver, thereby providing a metallic surface electrode having an intimate connection with the underlying portions of .the crystalline substance. In practicing the present invention, the entire opposite surfaces of each of the piezoelectric crystal layers 51, 52 may be coated with the graphite stratum, which does not add any stiffness to the crystal-layer members. However, the metallic coatings 56 and 57 are applied and united only to the opposite surfaces of the intermediate region 55 of the two crystal layers 51, 52 indicated by the border line 551 (Fig. so as to increase the stiffness of the intermediate region 55 of the transducer which is driven with maximum displacement.

In addition, the two piezoelectric layers 51, 52 are united to each other, as by a stratum? of cement, only at their far apart opposite ends 53 and along their wider intermediate region 55, indicated by the electrode boundary 551, so that the portions of the crystal layers 51, 52 extending between the boundaries 551of the facing intermediate surface electrodes 57 and the end portion 53 of the two layers 51, 52 are free frommechanical union and are free to slide past each other except for the' restaint exerted by the regions at which'they are mechanically affixed or joined to each other.

The intermediate surface region 55 and the end surface region 53 of the two piezoelectric layers 51, 52 are united toeach other by a fine film of cementitious material loaded with silver so as to providea good electric and mechanical connection between the facing electrode surfaces 57 provided along the central region 551 of the two piezoelectric layers. The mechanical junctions 53--1 between the far end portions 53 of the two piezoelectric layers 51, 52 may be provided by a similar cement, which does not have to be loaded with a silver since these end portions-do not have any active electrode surfaces.

As a further control of the stiffness, the electrode leads from the electrode surfaces of the bilaminate to the external circuits are so combined with the middle region of the bilaminate as to further increase the stiffness of the middle region and secure the desired equalization of the stresses over the entire length of the two laminations 51, 52. Thereis provided to this end, an upper electrode lead 67 and a lower electrode lead 68, each, for instance, in theform of a strip of thin metal foil, which are connected and united with their inner junction ends 69 to the opposite outer surface electrodes 56 of the bilaminate. The union of the lead 67, 68 to the electrode surfaces may beformed by a fine film of silver loaded cement. In the form shown in Fig. 4, the two inner junction'portions69 of the two electrode leads 67, 68 arelaterally displaced relatively to each other to provide stilfeningfor substantially the entire width ofthe middle-region 55 of the bilaminate-S0.- Alternatively, the two-foil strip leads 71, 72 may be positioned in overlapped relation over region of the outer electrodes 56, 57 of the bilaminate.

The bilaminate 50 shown is arranged to be provided with three mounting connections, one along portions of its transverse axis 601 and two along its far end portions 53, so that by connecting a vibratory force transmitting member to one of its three mounting connections while restraining the other two mounting connections, the bilaminate will operate as a mechanoelectric transducer having a force transmitting connection to an associated vibratory structure, such as vibratory diaphragm, phonograph stylus or the like.

In the microphone 30 of the invention shown, (Figs. 1, 2, 6), the opposite far end portions 53 of the bilaminate 50 are provided with mounting connector elements 61 through which they are connected to their restraining support while its intermediate bilaminate portion is provided with a driving connection to the vibratory diaphragm 35 for producing corresponding vibratory motion of the intermediate bilaminate portion 55 relative to its end portions 53 and generation of a corresponding volt age output.

In the form of microphone shown, an inner shallow metal casing 63, of sheet metal such as aluminum, is placed around the rear side and the borders of the piezoelectric bilaminate 50, being interposed therebetween and the rear wall of the microphone cavity casing 31.

The inner microphone casing 63 is suitably fixed within the surrounding borders of the microphone cavity casing 31 as by providing the flange 65 of the inner microphone casing 63 with four projecting pressure tabs 67 which engage and are retained with a wedge-like action within the four border corners of the microphone cavity casing. Alternatively, or in addition, the inner microphone casing 63 may be united to the outer cavity casing 31 by placing a thin layer of cement between abutting or contacting wall portions of the two casings.

The mounting connector elements 61through which the far end portions of the bilaminate 50 are restrained and connected to the inner casing wall 63 of the cavity casing 31-are formed by connector pads of stiff elastomer material such as durometer neoprene.

The intermediate portion 55 of the transducer bilaminate 50 is likewise provided with a mounting connection by which it is connected to the facing driving elements of the vibratory diaphragm 35.

The inner microphone casing 63 is shown provided with a generally rectangular border 64 having an outvvardly extending border flange 65 to which is secured, in a vapor-proof manner, a light, limp barrier diaphragm 66.

The limp barrier diaphragm may be formed of very thin metal foil, such as aluminum, of sufiicientdensity so that it forms the inner casing wall 63 a vapor-proof hermetically sealed transducer compartment within which the piezoelectric transducer 50 is enclosed'in a vaporproof manner to protect it against vapor and moisture. A good way to seal between the periphery ofthe limp barrier diaphragm 66' and the border flange 65 of the inner microphone casing 63 isto apply any of the available vapor-proof cementitious materials to'and coat there with the facing surfaces of the periphery of the barrier diaphragm 66 and the flange 65 of the inner casing 63 and thereafter pressing them towards each other until the cement is set.

According to the invention, the two transversely displaced end portions 54 of the piezoelectric bilaminate 50 are provided with mounting connections to facing portions of the driving vibratory microphone dia phragm 35.

The vibratory diaphragm 35 has a generally rectangu lar periphery which is alfixed 'as by'cement, to the out wafd flange 33 ofthe generally rectangularbordr wall 22 of the outer microphonecavity casing 21'. Therectangular border region of the diaphragm ;3,5 is separated mesa;

. I 7 from the oblate conical part 36 of the diaphragm by an endless rectangular corrugated region 391.

The vibratory diaphragm 35 is made of thin sheet material, for instance, metal foil of aluminum or the like. The rectangular diaphragm 35 is shaped into two stitfened oblong vibratory cone sections 36, each provided with an apex region 37 drivingly connected to the respective facing transverse end portions 54 of the intermediate section 55 of the bilaminate 50. The driving connection between the two driving portions 37 of the conical diaphragm regions 46 includes two metal clips 71 of sheet metal embracing and clamping to each other the opposite transverse end portions 54 of the intermediate region of the piezo-electric layers of the intermediate region of the bilaminate 50, a thin insulating separator, such as condenser paper or the like providing electric insulation between the clips 71 and the electrode surfaces of the bilaminate 50. The two metal clips 71 are also cemented to the transverse end portions 54 of the bilaminate 50, and to the insulator spacers which keep them insulated from the bilaminate body 50.

p The clamping metal clips 71 serve to affix to each other the intermediate vibratory portions 55 of the two piezo-electric layers 51, 52, and it is not necessary to provide an additional cementitious joint between the inner facing surface electrodes 57 of the two layers 51, 52. Furthermore, the inner surface electrodes 57 may extend over the entire area of each piezo-electric layer 51, 52, as these extensions do not substantially increase the stiffness of the end regions of the bilaminate.

The driving apex 37 of each of the two conical drive portions of the vibratory diaphragm 35 is connected to the driving portions 64 of the bilaminate 50, and the layers of cement applied between their facing portions and the interposed portions of the vapor-proof limp barrier diaphragm 66 so that the vibratory motion of the diaphragm 35 is effectively transmitted through their cone apexes 37 to the metallic clips 64 of the opposite transverse end portions 54 of the central region 55 of the bilaminate 50.

The terminal foil strips or leads 67, 68 are very soft and have connected thereto external leads 67-1, 68-1 of flexible insulated conductor wire enclosed in an insulating enclosure (Figs. 1, 2, 3). The external conductor leads 67-1, 681 are passed through small perforations in the rim 64 of the sealed inner microphone casing 63. The rim perforations of the inner casing 63 through which the two external leads 67-4, 68-1 pass, are hermetically sealed by applying thereto and to the adjoining lead portions an insulating sealing wax cement enclosure 69 of the type which provides the vapor-proof seal between the borders of the limp barrier diaphragm 36 and the rim flange 65 of the vapor sealed inner microphone casing 63.

With the moisture-susceptible piezoelectric transducer 50 thus completely enclosed in a vapor-proof compartment-formed by the inner metallic microphone casing 63 and the vapor-proof limp barrier diaphragm 66 interposed between the vibratory diaphragm and the piezoelectric element-moisture is kept out from the vaporproof compartment in which the piezoelectric bilaminate 50 will remain fully effective for a long useful life without materially losing its efliciency, as the limp barrier diaphragm 66 has only negligible stiffness and does not substantially impede the effective vibratory motion of the vibratory diaphragm 36 and the piezoelectric unit 50 driven thereby.

With such vapor-proof enclosure for the piezoelectric transducer element 50, the vibratory diaphragm may be provided with suitable vent hole openings 351 for venting the interior of the microphone cavity formed by the vibratory diaphragm 35 and the cavity casing wall 31 and thereby causing the cavity to resonate in the low frequency region of about 200 to 500 cycles, and raise the microphone sensitivity in this range.

Another phase of the invention involves a novel suspension of the microphone unit within the casing in which it is worn on the body of the user and arranged in such a manner as to suppress propagation of the very disturbing and annoying clothes-rubbing noises to the microphone cavity casing 31 and to its piezoelectric transducer unit 50.

According to the invention, the microphone is held sus ended in its operative position between opposite wall portions of a supporting structure by compliant suspension pads of elastic elastomer material, such as sponge rubber or foam rubber, which exert opposite suspension forces on the microphone structure and which also suppress the propagation of clothes-rubbing noises from the supporting structure to the microphone unit 30.

In accordance with the invention, the microphone unit 30 is provided with thin border elements such as thin outwardly extending flange elements of metal or like sheet material. Opposite peripherally spaced surface portions of these border sheet elements are engaged by compliant elastomer pads of material such as sponge rubber, foam rubber or the like which constitute the sole operative suspension of the microphone unit 30. The opposite elastomer pads are so arranged on the border elements of the microphone unit as to engage opposite wall portions of a microphone support structure in generally balanced manner as to exert on the microphone unit balanced opposite supporting forces which define the operating position in which the microphone unit is held by the elastomer pads within the supporting structure between its opposite supporting wall portions. Adjacent border elements of the microphone unit which are engaged by oppositely acting elastomer pads are laterally offset from the intermediate plane of the microphone support, thus making it possible to utilize thicker elastomer pads of greater volume as microphone suspension elements while keeping to a minimum the overall thickness of the space occupied by the microphone and its clothesrubbing noise suppressing suspension, each compressed elastomer pad being of greater thickness than the spacing between the plane of the microphone support and the opposite support structures engaged by the oppositely acting elastomer pads.

Figs. 1, 2 and 3 show one form of such microphone suspension based on the principles of the invention. The upper and lower rim flange portions 33 of the microphone casing 31 are shown provided with mounting extensions 71 offset against the flange portions 12 so as to occupy a position about mid-Way between the facing wall portions of the front wall 11 and rear wall 15 of the microphone housing which may be regarded as the suspension plane of the microphone 30.

Each of the opposite mounting portions 71 of the microphone casing is held suspended between opposite facing wall portions of the microphone casing walls 11 and 15 by two sets of opposite elastomer pads 73, 74 of suitable highly compliant elastomer material such as sponge rubber, foam rubber or the like. To provide opposite suspensions having each a substantial body or volume of elastomer material, each mounting sheet portion 71 is formed of oppositely otfset regions 71-1 of the microphone casing 71-2' so that each of the oppositely acting compressed elastomer pads 73, 74 is of substantial thickness and volume and is highly effective as a compliant elastomer body in suppressing and dampening noise vibrations propagated by the outer suspension walls toward the metallic mounting portions 71 of the microphone 30.

Referring to Figs. 1 to 4, the electrode leads 671, 631, which emerge at 69 from the vapor-proof sealed small apertures of the inner microphone casing 63 may be led along the exterior of the border rim portions 32 thereof toward the side of the microphone from which they are brought to the external circuits, such as the input stage of the amplifier housed in the amplifier casing 8.

In the form shown, the two emerging electrode leads 67-1, 68-4, are led through openings in the upper rim 32 of the outer microphone cavity casing 31 and along the recessed portions of the upper microphone mounting sheet 71 toward apertures in the upper region of the amplifier casing wall 9 into the interior space thereof.

Without thereby in anyway limiting its scope but only to enable ready practice of the invention, there are given below the principal data about a specific microphone of the type shown in Figs. 1 to 5.

The bilaminate 50 of Figs. 4, 5, was of Rochelle salt crystal material. It was about .750 inch long, its central portion was about .250 inch wide and about .030 inch thick. The microphone housing (with its housing walls 10, 15) was about .180 thick. The suspension pads were of foam rubber, and about .125 inch thick when not compressed, and about .095 inch thin in compressed condition shown.

The foregoing description of specific exemplifications all the invention will suggest to those skilled in the art various other modifications thereof. Thus, as shown in Fig. 7, a bilaminate generally similar to that of Figs. 5 and 6, may be formed of two similar piezoelectric layers 51, 52 joined along the entire inwardly facing electrode surfaces with the outer electrode surfaces provided along their intermediate region with generally diamond-like surface electrodes indicated by the border line 552.

Piezoelectric bilaminates of the invention may also be formed with three or more outwardly tapering arms similar to the two tapering arms of the bilaminate of Figs. 5 and 6, for instance, with four such arms extending from an intermediate region of the bilaminate structure constituting one of the force-coupling portions thereof, the other force-coupling portions being formed by the narrow end portions of the several arms corresponding to the end portions 53 of Figs. 5 and 6. Likewise, two substantially square piezoelectric layers may be joined in overlapping relation into a bilaminate structure by cementing or clamping them along the four overlapping corner portions and along the overlapping center portion of the two overlapping layers, leaving the regions of the two piezoelectric layers extending between their central portion and their four corner portion free from any mechanical union or junction along their inwardly facing surfaces.

In each of the foregoing transducers of the invention, either the central portion or the end portions of the transducer structure are provided with force coupling connectionsthrough which it is combined with other structures into a mechano-electric transducer structure. The force transmitting connection at the center of the transducer structure or at one of the end portionscorresponding to the central portion 55 and the end portions 53 of the transducer of Figs. 5 and 6may be used for coupling the piezoelectric bilaminate transducer to the external vibration transmitting member while the other force transmitting coupling portions of the bilaminate are restrained as by elastomers against vibratory movement.

The features and principles underlying the invention described above in connection with specific exemplifications, will suggest to those skilled in the art many other modifications thereof. It is accordingly desired that the appended claims be construed broadly and that they shall not be limited to the specific details shown and described in connection with exemplifications thereof.

I claim:

I. In an electro-mechanical transducer, an elongated piezoelectric transducer structure having a relatively thin transducer body of generally diamonddike quadrangular shape, two diagonally opposite corner end portions of said body defining a main transducing axis and having one set of two coupling elements, one for each corner portion, intermediate opposite lateral edge portions of said body defining a materially shorter transverse axis and araasaa having a. u t e s t o mai ng el ment o e; f r each lateral" edge portion, one set of Connector elements for connecting one of said two sets of coupling elements to one mass structure and a further set of connector elements for connecting the other of said sets. of coupling elements to a further mass structure, means for subjecting said thin body toa vibratory mode along said trans ducing axis, for generating. corresponding voltages when' vibratory driving forces are applied to one of said two sets of connector elements while the other set of connector elements is restrained against motion, the outer surface regions of said thin body adjoining said corner end portions being free of stiffening metallic electrode material, the outer surfaces of the intermediate region of said body including the lateral edge portions thereof having stiffening metal electrodes which impart materially greater stiffness to said intermediate region for materially equalizing the stresses to which the material of said body is subjected along different regions thereof when said body is performing electro-mechanical transducing operations with a vibratory mode along said transducing axis.

2. In an electro-mechanical transducer as claimed in claim 1, said thin transducer body comprising two body layers extending in overlapping relation adjacent to each other, said layers being united to each other along their inward facing surfaces only at their corner end portions.

3. In an electro-mechanical transducer as claimed in claim 1, said thin transducer body comprising two body layers extending in overlapping relation adjacent to each other, said layers being united to each other along their inward facing surfaces only at their corner end portions and at their lateral edge portions.

4. In an electro-mech'anical transducer as claimed in claim 1, said thin transducer body comprising two body layers extending in overlapping relation adjacent to each other, said layers being united to each other along their inward facing surfaces only at their corner end portions and at their intermediate region.

5. In an electromechanical transducer as claimed in claim 1, the mass structure which is connected through said coupling elements to portions of said transducer body extending along said transverse axis constituting a vibratory diaphragm through which said transducer body is acoustically coupled to the surrounding acoustic space.

6. In an electro-mechanical transducer as claimed in claim 1, said thin transducer body comprising two body layers extending in overlapping relation adjacent to each other, said layers being united to each other along their inward facing surfaces only at their corner end portions, the mass structure which is connected through said coupling elements to portions of said transducer body extending along said transverse axis constituting a vibratory diaphragm through which said transducer body is acoustically coupled to the surrounding acoustic space.

7. In an electro-mechanical transducer as claimed in claim 1, said thin transducer body comprising two body layers extending in overlapping relation adjacent to each other, said layers being united to each other along their inward facing surfaces only at their corner end portions and at their lateral edge portions, the mass structure which is connected through said coupling elements to portions of said transducer body extending along said transverse axis constituting a vibratory diaphragm through which said transducer body is acoustically coupled to the surrounding acoustic space.

8. In an electro-mechanical transducer as claimed in claim 1, said thin transducer body comprising two body layers extending in overlapping relation adjacent to each other, said layers being united to each other along their inward facing surfaces only at their corner end portions and at their intermediate region, the mass structure which is connected through said coupling elements to portions of said transducer body extending along said transverse axis constituting a vibratory diaphragm through which said transducer body is aeoustically coupled to the surrounding acoustic space.

References Cited in the file of this patent UNITED STATES PATENTS Semple Aug. 8, 1939 Pope May 20, 1941 

